The present invention relates to a Penning discharge plasma source. Before turning to the detailed description of the presently preferred embodiments, related prior art is discussed below. The related prior art is grouped into the following sections: magnetic confinement and the Penning cell source, facing target sputtering, and plasma treatment with a web on a drum. Other prior art methods and apparatuses are also discussed.
Magnetic Confinement and the Penning Cell Source
U.S. Pat. No. 2,146,025 to Penning discloses the first use of magnetic fields to enhance and confine a plasma for sputtering. The devices shown in Penning form the foundation for much of the future magnetically enhanced plasma source work. In several configurations shown, a magnetic field is used to extend the path length of electrons as they travel from the cathode to the anode. In extending the path length, the likelihood of a collision with a neutral atom and the creation of an ion and an additional electron increases. Penning uses this effect to achieve high rate sputtering of a cathode surface (or cathode erosion as he describes it). The source in FIGS. 5 and 6 in Penning is termed a Penning Cell or a Penning Discharge. Versions of this have been used to make ion beam sources, getter vacuum pumps, and other sources. In the figures, the magnetic field is a solenoid type field, stronger in the center of the gap than at the two cathode surfaces.
Magnetically confined plasmas are also discussed in J. Reece Roth, Industrial Plasma Engineering, Volume 1: Principles, IOP Publishing, Ltd. 1995. Section 9.5.5 pp 334-337 of this reference presents several Penning discharge configurations. One configuration, termed a Modified Penning Discharge, uses a mirror magnetic field to further improve the containment of electrons and ions. In Roth's work, sufficient magnetic field strength is used to contain ions as well as electrons.
Facing Target Sputtering
The following patents and reference disclose a method and apparatus implementing a sputtering technique where two cathodes face each other and a magnetic field is created normal to the surface of the cathodes: U.S. Pat. Nos. 4,407,894; 4,576,700; 4,767,516; 4,784,739; 4,842,708; 4,963,524; 5,000,834; 5,135,819; 5,328,583; 6,156,172; and Musil et al., Unbalanced magnetrons and new sputtering systems with enhanced Plasma ionization, American Vacuum Society, Journal of Vacuum Science and Technology A 9 (3) May/June 1991. In most of these documents, the substrate is placed outside of the gap between the cathode surfaces. For example, U.S. Pat. No. 4,767,516 to Nakatsuka et al. shows facing target sources used to coat web. The substrate is parallel to the magnetic field axis and outside the plasma region.
U.S. Pat. No. 4,963,524 to Yamazaki shows a method of producing superconducting material. An opposed target arrangement is used with the substrate positioned between the electrodes in the magnetic field. The difference is that the substrates shown here are in the middle of the gap. Testing shows this does not work well. The Hall current generated within the magnetic field tends to be distorted and broken when substrates are placed where shown in this patent. When this happens, the plasma is extinguished and/or the voltage is much higher.
In the article by Musii et al., several plasma sources are reviewed. Two figures, FIGS. 1e and f, show opposed target arrangements. In the text, it describes either placing substrates out of the plasma or close to the plasma depending upon the level of bombardment desired.
Plasma Treatment with a Web on a Drum
In U.S. Pat. Nos. 5,224,441 and 5,364,665 to Felts et al., a flexible substrate is disposed around an electrified drum with magnetic field means opposite the drum behind grounded shielding. In this arrangement, the shield opposite the drum is either grounded or floating. The magnet and electrode configuration also does not lend itself to effectively contain electron Hall currents. Note that in this patent, as the shield surface is coated, the overall system capacitance decreases, changing the circuit impedance. Shield coating is also a maintenance problem in a production operation. To exemplify the scale of the potential shield coating, if a 90 cm diameter roll of 12 um polymer film (approximately 50,000 meters of film) is coated with a 30 nanometer thick coating, assuming the shield opposite the substrate receives an equal amount of coating, then the shield will collect a >2 mm thick polymer coating by the end of the run.
In U.S. Pat. No. 6,110,540 to Countrywood et al., a disclosure is made regarding the electrified drum technology referenced above. The problem of electrode coating is acknowledged in this patent. A method to maintain the conductivity of the non-drum electrode is disclosed. One issue with this solution is that, since polymerization occurs wherever there is glow, the glow at the gas feed electrode will be a high rate polymerization site causing additional maintenance headaches.
The problem of coating non-substrate surfaces is addressed in U.S. Pat. No. 4,863,756 to Hartig et al. In this disclosure, the substrate is continuously moved over a sputter magnetron surface with the surface facing the magnetron located inside the dark space region of the cathode. In this way, the magnetic field of the magnetron passes through the substrate and is closed over the substrate surface constricting the plasma onto the surface. The problem with this method is the substrate on the magnetron effectively becomes a sputtering target and receives intense ion bombardment. Therefore, while a PECVD film is being deposited on the substrate, the intense ion bombardment is simultaneously ablating the film. Another difficulty is that due to the intense ion bombardment, the substrate can become very hot. This limits substrate materials options and requires the substrate move at a speed sufficient to prevent damage from overheating. This limits the application options for this device.
Other Prior Art Methods and Apparatus
U.S. Pat. No. 4,761,219 to Sasaki et al. shows a magnetic field passing through a gap with the wafer on one electrode surface. In this case, the electrodes are opposed to each other.
U.S. Pat. No. 5,099,790 to Kawakami shows a microwave source with a moving magnet below the wafer to even out the coating on the wafer. In another figure, the substrates are moved over a stationary magnet(s). This source does not have opposed cathode (electrode) surfaces.